1. Field of the Invention
This invention pertains generally to liners in pressure vessels, more particularly to metal liners with improved pressure cycle life in composite pressure vessels, and still more particularly to metal liners incrementally autofrettaged for improved pressure cycle life in composite pressure vessels.
2. Description of Related Art
Many high performance composite pressure vessels are fabricated with metal liners to provide vessels with higher temperature performance, to contain reactive materials, or to simplify manufacturing. Metal lined composite pressure vessels have been in use for over fifty years.
One of the issues with metal liners relates to their fatigue properties. Pressure cycling of a metal liner eventually leads to metal fatigue, cracking, and failure. Metal lined composite pressure vessels may use a process called autofrettage to improve the metal liner stress profile and corresponding cycle life.
Autofrettage was originally used by the French to extend the life of metal gun barrels by internally pressurizing the barrel beyond the yield strength of the inner portion of the barrel so that it would plastically yield. When depressurized, the yielded material would then equilibrate in a state of partial compression. Generally cracks cannot form or grow if a metal is in compression. Therefore, when the gun is fired, the inner portion of the gun barrel would not reach as high a tensile stress state as it otherwise would have if not autofrettaged, because it began pressurization in a compressive stress state. Therefore, autofrettaged gun barrels lasted longer than untreated gun barrels.
This same autofrettage procedure may be used in metal lined composite pressure vessels to place the liner in compression at ambient (zero gage) pressure. As the vessel is pressurized during use, the liner does not reach the stress state it otherwise would have, had the autofrettage process not been used.
During the initial autofrettage pressurization of a metal lined composite pressure vessel, locally high plastic strains can occur at:
(1) stress concentrations, such as transitions from thin to thick regions;
(2) welds, where the joined materials may have initially different amounts of cold work and the yield strength may be lower or higher than the mating material or the weld itself; or
(3) at locally thin areas or defects.
In extreme cases, these local high plastic strain areas will crack during the autofrettage cycle. Alternatively, such areas may buckle during depressurization due to the induced compressive stresses and locally reduced stiffness due to local yielding. In less severe cases, the liner will prematurely fail due to cyclic fatigue at such locations.
The only way previously used to prevent premature failures when using the single cycle autofrettage process was to limit local plastic strains by increasing the composite overwrap thickness profile, or to generally increase liner thickness in such a weak area. However, such additional thickness increases both pressure vessel weight and cost. Changing the liner thickness profile may take significant time and may not be compatible with the manufacturing process or required end product.